Molecular Playground: Hexameric ClpX

ClpX is a CBI Molecules being studied in the Chemistry-Biology Interface Program at the University of Massachusetts Amherst. It will be on display at Molecular Playground, with the banner "ClpX ‘spring cleans’ by dumping some proteins and refolding others." 

Introduction
ClpX belongs to the AAA+ (ATPases Associated with diverse cellular Activities) family of proteins that are able utilize chemical energy obtained from ATP hydrolysis for their mechanical activity. ClpX is known to function as a molecular chaperone that unfolds native proteins, and as a protein degradation machinery when it forms a complex with the protease ClpP.

In the cell, ClpX forms a ring homo-hexamer that resembles a donut (electron micrographs). The top of the ring contain regions that recognize specific proteins. These proteins are unfolded through the central pore, and sent out of the bottom of the ring.

The crystal structure of hexameric ClpX without ATP and with ATP from Escherichia coli was solved in 2009. Before that, there was only the crystal structure of monomeric ClpX from Helicobacter pylori, therefore information about the mechanistic abilities of hexameric ClpX was limited.

Structure
Notably, hexameric ClpX has a highly  conserved central pore and conserved interface between subunits/chains. The central pore contain pore loops that help transport proteins,. The staggered subunits are approximately 2-fold rotational symmetric. Each monomeric subunit consist of a large AAA+ domain, small AAA+ domain, and a linker that joins the two domain.

Even though the structure is literally a homo-hexamer, the subunits are structurally heterogenous. Two distinct types of subunits are known as Type-1 subunits where ADP nucleotides were found, and Type-2 subunits which do not contain ADP nucleotides. There are altogether four type-1 subunits and two type-2 subunits in a hexameric ClpX, fashioned in a cyclic 1-1-2-1-1 manner. In sum, there are only four sites for ATP/ADP nucleotide binding, instead of six sites given that there are six ClpX monomers units, and this is not surprising because of previous experimental findings that ClpX hexamer bind a maximum number of four ATP only.

The positioning of Leucine 317 in the linker region is responsible for ADP (or ATP) binding. The leucine 317 in Type-1 subunits contacts the adenine base of the nucleotide ADP, while the leucine 317 in Type-2 subunits faces the opposite direction, causing conformation changes that prevents ADP binding.

Interestingly, the large domain in the two types of subunits are structurally similar, however structural variability is obvious in the small domain (Type-1 subunit | Type-2 subunit

Taken together, these structure characteristics attribute to the mechanistic motion of hexameric ClpX (restore initial morphing movie ).

My research interest
To further define the specificity of ClpX function and determine its influences beyond cell cycle regulation.

Acknowledgment
Emeritus Professor Eric Martz, for his kind consultation.